This invention relates in general to electrophotographic imaging members and more specifically, to imaging members comprising titanyl phthalocyanine and charge transporting polymer components.
One common type of electrophotographic imaging member or photoreceptor is a multilayered device that comprises a conductive layer, an optional charge blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. Either the charge generating layer or the charge transport layer may be located adjacent the conductive layer. The charge transport layer can contain an active aromatic diamine small molecule charge transport compound dissolved or molecularly dispersed in a film forming binder. This type of charge transport layer is described, for example in U.S. Pat. No. 4,265,990. Although excellent toner images may be obtained with such multilayered photoreceptors, it has been found that when high concentrations of active aromatic diamine small molecule charge transport compound are dissolved or molecularly dispersed in a film forming binder, the small molecules tend to crystallize with time under conditions such as higher machine operating temperatures, mechanical stress or exposure to chemical vapors. Such crystallization can cause undesirable changes in the electro-optical properties, such as residual potential build-up which can cause cycle-up. Moreover, the range of binders and binder solvent types available for use during coating operations is limited when high concentrations of the small molecules are sought for the charge transport layer. For example, active aromatic diamine small molecules do not disperse in polyurethane binders. Limited selection of binders and binder solvents can affect the life and stability of a photoreceptor under extended cycling conditions. Moreover, such limited selection also affects the choice of binders and solvents used in subsequently applied layers. For example, the solvents employed for subsequently applied layers should not adversely affect any of the underlying layers. This solvent attack problem is particularly acute in dip coating processes. Further, some of the solvents that are commonly utilized, such as methylene chloride, are marginal solvents from the point of view of environmental toxicity. Although excellent toner images may be developed with multilayer photoreceptors in machines that employ dry developer powder or toners, it has been found that these same photoreceptors become unstable when employed with liquid development systems. These photoreceptors suffer from cracking, crazing, extraction, phase separation and crystallization of charge transporting active compounds by contact with the organic carrier fluid in a machine employing a liquid development system. A commonly employed organic carrier fluid in liquid development systems is an isoparaffinic hydrocarbon, for example, Isopar.RTM. available from Exxon Chemicals International, Inc. The leaching and crystallization of charge transporting active compounds markedly degrades the mechanical integrity and electro-optical performance of the photoreceptors. More specifically, the organic carrier fluid of a liquid developer leaches out activating small molecules, such as the arylamine containing compounds typically used in the charge transport layers. Representative of this class of materials are: N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[1,1' -biphenyl]-4,4'-diamine; bis-(4-diethylamino-2-methylphenyl)-phenylmethane; 2,5-bis-(4'-diethylamino phenyl)-1,3,4-oxadiazole; 1-phenyl-3-(4'-diethylaminostyryl)-5-(4"-diethylaminophenyl)-pyrazoline; 1,1-bis-(4-(di-N,N'-p-methylphenyl)-aminophenyl)-cyclohexane;4-diethylamin obenzaldehyde-1,1-diphenylhydrazone; 1,1-diphenyl-2(p-N,N-diphenylamino phenyl)-ethylene. The leaching process results in crystallization of the charge transporting activating small molecules, such as the aforementioned arylamine compounds, onto the photoreceptor surface and subsequent migration of the arylamine into the liquid developer ink. In addition, the ink vehicle, typically a C.sub.10 -C.sub.14 branched hydrocarbon, induces the formation of cracks and crazes in the photoreceptor leading to the onset of copy defects and shortened photoreceptor life. Sufficient degradation can occur in less than eight hours of use making these photoreceptors unsuitable for use in machines employing liquid developers.
Another type of charge transport layer has been developed which utilizes a charge transporting polymer. This type of charge transport polymer includes materials such as poly N-vinyl carbazole, polysilylenes, and others including those described in U.S. Pat. Nos. 4,806,443, 4,806,444, 4,818,650, 4,935,487, and 4,956,440. Other charge transporting materials include polymeric arylamine compounds and related polymers described in U.S. Pat. Nos. 4,801,517, 4,806,444, 4,818,650, 4,806,443, and 5,030,532, copending application Ser. No. 797,753, entitled "ELECTROPHOTOGRAPHIC IMAGING MEMBER", mailed by Express Mail on Nov. 25, 1991, in the name of Yanus et al, and copending application Ser. No. 798,363, entitled "ELECTROPHOTOGRAPHIC IMAGING MEMBERS CONTAINING POLYARYLAMINE POLYMERS", mailed by Express Mail on Nov. 25, 1991, in the name of Yanus et al, the disclosures of which are incorporated herein by reference in their entirety. Some polymeric charge transporting materials have relatively low charge carrier mobilities. Mechanical properties of these pendant type polymers, such as poly N-vinyl carbazole and polystyryl anthracene, is less than adequate for photoreceptor belt fabrication and operation. Moreover, charge transporting polymers having high concentrations of charge transporting moieties in the polymer chain can be very costly. Further, the mechanical properties of charge transporting polymers such as wearability, hardness and craze resistance are reduced when the relative concentration of charge transporting moieties in the chain is increased.
Phthalocyanines have been employed as photogenerating materials for use in both visible and infrared radiation exposure machines. Infrared sensitivity is a requirement if semiconductor lasers are employed as the exposure source. The absorption spectrum and photosensitivity depend on the central metal atom. Many metal phthalocyanines have been reported. These include, oxyvanadium phthalocyanine, chloroaluminum phthalocyanine, copper phthalocyanine, oxytitanium phthalocyanine, chlorogallium phthalocyanine, magnesium phthalocyanine and metal-free phthalocyanine. Some of these phthalocyanines exist in many crystal forms. Even with the same central metal atom, the absorption spectrum and sensitivity may depend on crystal structure and morphology.
The photogenerating layer contains a bichromophoric photogenerating compound, for example a phthalocyanine pigment compound, or a mixture of two or more phthalocyanine pigment compounds. Generally, this layer has a thickness of from about 0.05 micrometer to about 10 micrometers or more, and preferably has a thickness of from about 0.1 micrometer to about 3 micrometers. The thickness of this layer, however, is dependent primarily upon the concentration of photogenerating material in the layer, which may generally vary from about 5 to 100 weight percent. When the photogenerating material is present in a binder material, the binder preferably contains from about 30 to about 95 percent by weight of the photogenerating material, and preferably contains about 80 percent by weight of the photogenerating material. Generally, it is desirable to provide this layer in a thickness sufficient to absorb about 90 percent or more of the incident radiation which is directed upon it in the imagewise or printing exposure step. The maximum thickness of this layer is dependent primarily upon factors such as mechanical considerations, such as the specific photogenerating compound selected, the thicknesses of the other layers, and whether a flexible photoconductive imaging member is desired.
The sensitivity of a layered device depends on several factors: (1) the fraction of the light absorbed, (2) the efficiency of photogeneration within the pigment crystals, (3) the efficiency of injection of photogenerated holes into the transport layer and (4) the distance the injected carrier travels in the transport layer between the exposure and development steps. The fraction of the light absorbed can be maximized by the employment of adequate concentration of pigment in the generator layer and the thickness of the generator layer. The distance the carrier travels in the transport layers can be optimized by the selection of the transporting material and on the concentration of the charge transporting active molecules in the case of transport layers consisting of a dispersion of transport active molecules in a non-transporting inactive binder. However the efficiency of photogeneration and injection can be interactive in that both processes depend on both the pigment and the transport material. There are at least two reasons for this interactive dependence. The photogeneration efficiency with some pigments depends upon the presence of the transporting material on the surface of the pigment. Devices fabricated employing these pigments may be sensitive with transport layers employing active molecules dispersed in an inactive binder material but may be very much less sensitive when employed in conjunction with transport layers consisting of charge transporting polymers. This dependence arises in the case where the transport layer consists of active molecules dispersed in an inactive binder (herein termed small molecule transport layer), from the active molecules penetrating the generator layer during the fabrication of the transport layer. This is not the case when the transport layer consists of a charge transporting polymer. Therefore there is no certainty that a pigment that seems sensitive in a device employing small molecule transport layer will have good sensitivity when employed in conjunction with a charge transporting polymer. Interactive dependence of injection efficiency can also be related to ionization potential matching of the constituent charge transport molecules and the charge generating pigment or pigments. For layered devices employing hole photogeneration and transport, the ionization potential of the charge transport layer material (IP.sub.CTL) has to be smaller than the ionization potential of the charge generating pigment (IP.sub.CGP) to ensure maximum injection efficiency. That is, IP.sub.CTL &lt;IP.sub.CGP.
Thus, in imaging systems utilizing multilayered photoreceptors containing generator layers employing some pigments and charge transporting polymers in the transport layers, loss of sensitivity may result from the active transport species not physically penetrating the generator layer or as a result of an ionization potential mismatch. Reduced sensitivity can reduce the practical value of multilayered photoreceptors for use in high speed electrophotographic copiers, duplicators and printers.